The present invention relates to a vaporized metal discharge lamp of the type wherein mercury, inert gas and metal halides are filled in a transparent light emission envelope having electrodes mounted thereon, and more particularly to a method of filling metal halides in the envelope.
Generally, metals such as thallium, sodium, indium and the like are filled in the form of metal halides in the light emission envelope of a high pressure mercury discharge lamp, to improve light emission efficiency, color rendition and the like. However, of the metal halides, there is used a substance which has a high moisture absorption characteristic, such as indium iodide so that it cannot be exposed to atmospheric air. Thus, the method of manufacturing a vaporized metal discharge lamp has been made complicated.
According to a most common manufacturing method, granular metal halides previously dehydrated and weighed are first filled in a light emission envelope within a dry box under an inert gas atmosphere. Next, the light emission envelope is air tightly sealed from external atmospheric air by proper means and coupled to an air exhaust device to exhaust air from within the light emission envelope. Thereafter a series of processes including mercury dropping, inert gas introducing, and sealing are carried out, thus resulting in a complicated manufacturing procedure.
In Japanese Patent Publication Nos. 40-19548, 43-17787, 46-19390 and others there has been proposed a manufacturing method whereby a metal halide pool is formed at the air exhaust pipe or at the sealing member, and the metal halide is heated while exhausting air via the air exhaust pipe to dehydrate it and vaporize it so as to make it easy to be moved. Then, such metal halide is introduced into the light emission envelope previously cooled, and is concentrated within the envelope.
Further, in Japanese Patent Publication No. 54-14874, a manufacturing method has been proposed whereby a metal halide dropping device is mounted in the air exhaust system to drop it into the light emission envelope.
In spite of the complicated and inefficient conventional method described above, it is still difficult to completely remove water, oxygen and the like adsorbed on the surfaces of jigs and the like within the dry box, and of jigs and the like for maintaining air tightness of the light emission envelope. These substances are adsorbed by the metal halide, resulting in the disadvantages such as an extraordinarily high discharge start voltage, early blackening of the envelope, and inactivation of the lamp. These disadvantages have not been eliminated to date.
According to the manufacturing method disclosed in Japanese Patent Publication Nos. 40-19548, 43-17787, and 46-19390, the metal halide does not contact the atmospheric air so that the conventional disadvantages such as an extraordinarily high discharge voltage, early blackening of the envelope, and inactivation of the lamp have been eliminated. However, it has been found that there arises a new problem in that it is very difficult to fill reliably a predetermined amount of metal halide in the light emission envelope. In particular, even if a metal halide precisely weighed is placed in the metal halide pool, the vapor of heated metal halide will be concentrated, during transfer thereof, at the low temperature areas not only within the light emission envelope but also within an introduction pipe, air exhaust pipe and the like. Thus, all the metal halide previously weighed cannot be concentrated within the light emission envelope. The fluctuation of the filling amount of metal halide becomes more conspicuous as the light emission envelope becomes smaller. The fluctuation of the filling amount of metal halide directly influences the discharge characteristic of the lamp so that this manufacturing method leaves a significant problem.
The manufacturing method proposed in Japanese Patent Publication No. 54-14874 also has a disadvantage which cannot be overlooked. In particular, first, as the discharge lamp becomes smaller, the filling amount of a metal halide also becomes less so that the size of the metal halide becomes not easy to be dropped. In addition, it has been found that even if it is possible for the metal halide to be dropped by all means, it collides with or attaches to the wall of the light emission envelope while dropping into the envelope so that the amount of the metal halide reaching the inside of the envelope is reduced and also fluctuates to a large extent.
Therefore, a discharge lamp manufactured by this method has a fluctuation of the filling amount of the metal halide so that the fluctuation of the discharge characteristic cannot be avoided, similar to the discharge lamp manufactured by the former methods. Further, according to the above methods heretofore proposed, it is necessary to weigh the metal halide for each discharge lamp beforehand. This weighing work is also required to be carried out within the dry box and is very inefficient. Furthermore, since several types of metal halides are mixed and used in general, not only does the number of weighing operations become large, but also the amount of each metal halide becomes very small. A precise weighing operation for such a minute amount has been required. However, in practice, both a high work efficiency and weighing precision have been impossible to be realized at the same time.